The present invention relates to an improved resin-coated sand to be used in a shell-molding process. In conventional sand-molding operations, a mixture of sand coated with binder is placed in the mold, and the heat of the processing steps causes reactions to occur between the binder components to improve the pressed strength of the sand and retain the configuration of the part to be cast. After introduction of the molten metal into the cavity, the heat of the metal, during the cooling cycle is transferred to the sand-binder mixture causing the binder to be destroyed to a degree that allows the sand to be removed from the cast metal in an efficient manner.
In the automobile industry, the trend of manufacturers has led to the replacement of iron and steel castings with lighter weight metals such as aluminum, magnesium and their alloys. These castings are produced by sand-molding processes, but occur at lower temperatures than iron castings. The use of conventional binders, at these lower temperatures, have created problems in the removal of the sand particles from the castings due to the failure of the binder to be decomposed.
In the case of iron casting, the stock temperature of shell-mold reaches 800.degree.-1000.degree. C. at pouring, and the strength of shell-mold is naturally reduced after casting because almost all the phenolic resin binder is subjected to thermal degradation by the intense heat at pouring. Accordingly it is easy to remove the mold-core from molded articles in the form of sand grains after casting.
For metals having a lower melting temperature, such as aluminum and magnesium, the stock temperature of shell-mold at pouring is rather low, approximately 300.degree.-400.degree. C. This results in an incomplete thermal degradation of the phenolic resin binder. Since conventional shell-molds have retained superfluous strength after casting for this reason, there have been extreme difficulties particularly for complicated mold structures, in removing the core efficiently from molded articles. In these cases, flogging is required so as to crush the molds even after time-consuming calcination thereof in a furnace to remove the occulded core therefrom. Flogging is a term used to indicate a tapping or impact force applied to the castings to remove the particulate sand particles leaving a clean cast structure.
After much investigation to improve the shake-out property of shell-molds after casting metals having a lower melting temperature, such as aluminum, the inventors have found that the shake-out property of cast shell-molds is greatly improved by using a resin-coated sand produced by coating foundry sand with a phenolic resin in the presence of a carbonate or bicarbonate (hereafter abbreviated carbonates) of elements selected from groups Ia, IIa, IIIb, IVb, Vb, VIb, VIIb and VIII of the Periodic Table of Elements.
It is an object of this invention to provide a resin binder for shell-molding operations that will provide a coated foundry sand possessing sufficient strength for casting molten metals, such as aluminum, magnesium and their alloys, and upon cooling of the casting, will allow easy removal of the sand from the cast object.
It is an additional object of this invention to provide a resin binder for foundry sand that upon completion of the molding cycle will be sufficiently degraded or decomposed that the sand can be reused easily in subsequent molding operations.